In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method which comprises rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink unreceptive area), thereby making a difference in adherence of ink on the surface of the lithographic printing plate, and depositing the ink only to the image area by utilizing the nature of the dampening water and oily ink to repel with each other, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the image-recording layer to form the image area and removing other unnecessary image-recording layer by dissolving with an alkaline developer or an organic solvent thereby revealing the hydrophilic surface of support.
In the hitherto known plate-making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary image-recording layer by dissolving, for example, with a developer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate-making methods in response to the above-described requirement, it has been proposed a method referred to as on-machine development wherein a lithographic printing plate precursor having an image-recording layer the unnecessary portion of which is capable of being removed in a conventional printing process is used and after exposure, and the unnecessary portion of the image-recording layer is removed on a printing machine to prepare a lithographic printing plate.
Specific methods of the on-machine development include, for example, a method of using a lithographic printing plate precursor having an image-recording layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image-recording layer by contact with rollers or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image-recording layer or adhesion between an image-recording layer and a support upon penetration of dampening water, ink solvent or the like and then mechanically removing the image-recording layer by contact with rollers or a blanket cylinder of a printing machine.
In the invention, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing an infrared laser-unexposed area of the image-recording layer of the lithographic printing plate precursor upon contact with liquid (ordinarily, an alkaline developer) thereby revealing a hydrophilic surface of support. The term “on-machine development” means a method and a step of removing an infrared laser-unexposed area of the image-recording layer of the lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitalized image information on highly converging radiation, for example, laser light and conducting scanning exposure of a lithographic printing plate precursor to the light thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
As described above, in recent years, the simplification of plate-making operation and the realization of dry system and non-processing system have been further strongly required from both aspects of the consideration for global environment and the adaptation for digitization.
In general, an operation for inspection and discrimination of image formed on a printing plate is carried out in order to examine whether the image is recorded on the printing plate as intended, in advance of mounting the printing plate on a printing machine. In a conventional lithographic printing plate precursor subjected to the development processing step, it is ordinarily easily performed to confirm the image formed after the plate-making (after the development processing) and before the printing (before the mounting the printing plate on a printing machine) by means of coloration of the image-recording layer.
However, with respect to the lithographic printing plate precursor of the on-machine development type or non-processing (non-development) type without accompanying the development processing prior to printing, the image is not recognized on the printing plate in the step of mounting it on a printing machine, and thus the discrimination of the printing plate can not be performed. In particular, it is important in the printing operation to determine whether a register mark which acts as a landmark for the registering in multicolor printing is recorded. Therefore, in the lithographic printing plate precursor of the on-machine development type or non-processing (non-development) type, a means for confirming the image (print-out agent) at the stage of exposure or heating is required.
Printing plates have been proposed wherein a compound capable of generating an acid, base or radical by means of light or heat and a compound capable of undergoing color change upon interaction with the acid, base or radical generated are used as the print-out agent (for example, see JP-A-11-277927 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). Also, it has been proposed to utilize color change of thermally decomposable compound as the print-out agent of a direct-drawing type lithographic printing plate precursor having a heat-sensitive layer (for example, see JP-A-2000-335129). Further, it has been proposed to use a thermally decomposable dye having a decomposable temperature of 250° C. or below as the print-out agent (for example, see JP-A-2003-191657).